In the area of industrial metrology, a multitude of applications exist, where radiometric measuring devices have to be operated in locations that are exposed to a risk of explosion. Filling stations and chemical plants can be mentioned here as examples, where explosive gases can form.
Devices that are usable in explosion-prone areas have to comply with very strict safety requirements. Said requirements have the objective of preventing or avoiding spark formation, which could potentially trigger an explosion, since a spark that occurs inside an enclosed space can still have an impact on the surroundings. This objective can be achieved in different ways. The respective explosion protection measures are subdivided into ignition protection classes that are regulated in the respective industry standards, as in, for example, the series of standards IEC 60079 of the International Electrotechnical Commission.
The ignition protection class that is particularly relevant for measuring devices in practice is “inherent safety (Ex-i).”
For devices designed according to the ignition protection class “inherent safety,” the values for the existing electrical energy, reflected particularly in the electrical quantities of current, voltage, and power, have to be below a preset limit value inside the device at all times. The limit values are selected such that in the event of a failure, e.g., a short circuit, the maximum released energy is not sufficient to create an ignition spark or a dangerous overheating. Usually, the amount of energy is kept below the preset limit values by way of power-limiting measures, e.g., by respective combinations of current-limiting and voltage-limiting components. In addition to that, depending upon the device design, minimum safety distances or insulation measures between the individual components or circuits have to be provided.
In cases where energy cannot be limited to inherently safe values, additional ignition protection measures of other ignition protection classes have to be applied, e.g., ignition protection measures of the ignition protection class “pressurized enclosure (Ex d),” the ignition protection class “encapsulation (Ex m),” or the ignition protection class “powder filling (Ex q).” This requires relatively elaborate mechanical measures.
Detectors provided in radiometric measuring devices for detection of ionizing radiation regularly require very high operating voltages—particularly, voltages in the kilovolts range. Classical examples are Geiger-Müller counters and scintillator detectors, where gamma radiation incident on a scintillator is converted into light pulses which then are subsequently converted into electrical signals by a photomultiplier that is operated with high voltage.
In order to be able to use such measuring devices in explosion-prone areas, their components that require high voltage for operation are nowadays regularly arranged inside an encapsulation or in a pressure-tight enclosure. In addition, these components have to be sufficiently electrically insulated and positioned at a sufficiently distant location from the rest of the measuring device components, in order to prevent a potential flashover of the high voltages onto unsecured areas—particularly, onto inherently safe measuring circuits. Thus, in addition to the ignition protection class measures of “inherent safety (Ex-i),” measures of other ignition protection classes—particularly, complex encapsulations—always need to be applied.
These parallel measures to be provided are not only cost-intensive, but also require a lot of space. Thus, such measuring devices can be used only where sufficient space for the measuring device is available at the measuring location.